This invention relates to a device for producing a control signal for feedback control of the air/fuel ratio of an air-fuel mixture supplied to a combustor, such as the combustion chambers of an internal combustion engine, based on the concentration of oxygen in a combustion gas exhausted from the combustor.
In recent internal combustion engines, particularly in automotive engines, there has been developed a marked tendency to very minutely control the air/fuel mixing ratio to improve the efficiencies of the engines and reduce the emission of noxious components of exhaust gas. In many cases it is desired to feed an engine with a stoichiometrical air-fuel mixture, and it has already been put into practice to perform feedback control of air/fuel mixing ratio with the aim of maintaining a stoichiometric air/fuel ratio by using an exhaust gas sensor which provides a feedback signal indicative of the composition of an air-fuel mixture actually supplied to the engine.
For example, in an automotive engine system using a so-called three-way catalyst which can catalyze both reduction of nitrogen oxides and oxidation of carbon monoxide and unburned hydrocarbons contained in the exhaust gas, it is quite important to feed the engine always with an exactly stoichiometrical air-fuel mixture because this catalyst fully exhibits its ability in an exhaust gas produced by combustion of a stoichiometrical air-fuel mixture. Accordingly, in this engine system it becomes indispensable to perform feedback control of the air/fuel mixing ratio.
Usually, conventional feedback air/fuel ratio control systems aiming at a stoichiometric air/fuel ratio utilize an oxygen sensor that operates on the principle of concentration cell as an exhaust gas sensor to provide a feedback signal. This type of oxygen sensor has a layer of an oxygen ion conductive solid electrolyte, such as zirconia stabilized with calcia, formed into the shape of a tube closed at one end, a measurement electrode layer porously formed on the outer side of the solid electrolyte tube and a reference electrode layer formed on the inner side of the tube. When there is a difference in oxygen partial pressure between the reference electrode side and measurement electrode side of the solid electrolyte layer, this sensor generates an electromotive force between the two electrode layers. As an exhaust gas sensor, the measurement electrode layer is exposed to an engine exhaust gas while the reference electrode layer on the inside is exposed to atmospheric air utilized as the source of a reference oxygen partial pressure. In this state the magnitude of an electromotive force generated by this oxygen sensor exhibits a great and sharp change between a maximally high level and a minimally low level upon the occurrence of a change in the air/fuel ratio of a mixture fed to the engine across the stoichiometric air/fuel ratio. Accordingly it is possible to produce a fuel feed rate control signal based on the result of a comparison of the output of the oxygen sensor with a reference voltage which is set at the middle of the high and low levels of the sensor output.
However, this type of oxygen sensor has disadvantages such as significant temperatures dependence of its output characteristic, necessity of using a reference gas such as air, difficulty in reducing the size and insufficiency of mechanical strength.
To eliminate these disadvantages, U.S. patent application Ser. No. 12,763 filed Feb. 16, 1979 and now U.S. Pat. No. 4,207,159 discloses an advanced oxygen sensor, which is of a concentration cell type having a flat solid electrolyte layer with reference and measurement electrode layers formed respectively on the two opposite sides thereof and a shield layer formed on the reference electrode side of the solid electrolyte layer so as to cover the reference electrode layer entirely. Either the shield layer or the solid electrolyte layer is made rigid and thick enough to serve as a substrate, and each of the remaining three layers can be formed as a thin film-like layer. This sensor does not use any reference gas. Instead, a DC power supply is connected to this sensor so as to force a current to flow through the solid electrolyte layer between the reference and measurement electrode layers thereby to cause migration of oxygen ions through the solid electrolyte layer in a sensed direction and, as a consequence, establish a reference oxygen partial pressure at the interface between the solid electrolyte layer and the reference electrode layer. (The particulars of this oxygen sensor will be described hereinafter.)
When disposed in an engine exhaust gas, this advanced oxygen sensor exhibits an output characteristic generally similar to that of the conventional oxygen sensor having a tubular solid electrolyte. Accordingly, this oxygen sensor is serviceable as a device to provide a feedback signal in an air/fuel ratio control system aiming at a stoichiometric air/fuel ratio. Moreover, this sensor has advantages such as unnecessity of using any reference gas, possibility of making small in size and good resistance to shocks and vibrations. However, the output characteristic of this oxygen sensor too is significantly affected by temperature. Particularly, when the temperature of the sensor is below about 500.degree. C. the output characteristic changes such that it becomes difficult to make a comparison between a reference voltage of an adequate level and the output of the sensor. This is a matter of great inconvenience in practical air/fuel ratio control systems for automotive engines.